1. Field of the Invention
The present invention relates to an apparatus and method for reducing nitrogen oxide (hereinafter referred to as NOX) emissions from a furnace that is vertically fired from multi-nozzle, inter-tube burners located in the furnace roof. This method of reducing NOX involves off-stoichiometric combustion to reduce the formation of NOX. In particular, the present invention relates to the use of combustion system modifications, such as blocking or eliminating at least some nozzles of a coal burner, to achieve higher coal to air ratios during initial combustion.
2. Description of the Prior Art
NOX emissions from combustion devices are a major regulatory concern in many industrialized countries. Nitric oxide (hereinafter NO), which is the usual form of NOX emitted from furnaces, is converted to nitrogen dioxide (hereinafter NO2) in the atmosphere in a matter of a few hours or days after emission. NOX emissions are currently the subject of strict regulatory control. Among the objectives of these regulations are: reduction of acid rain, reduction of smog, reduction of eye and respiratory irritation, and reduction of formation of ozone. Some laws and regulations governing NOX emissions have been in force for 25 years. Additionally, even more stringent regulatory control will become effective after 1995.
Empirical studies have identified two mechanisms for the formation of NOX in pulverized coal-air flames: (1) thermal reaction of nitrogen and oxygen contained with combustion air to form NOX (hereinafter thermal NOX), and (2) the oxidation of organically bound nitrogen compounds contained within coal to NOX (hereinafter fuel NOX). For conventional furnaces, thermal NOX formation becomes significant at temperatures above 2800 degrees Fahrenheit. Conversion of fuel-bound nitrogen to NOX can occur at much lower temperatures. Empirical studies have revealed that fuel NOX represents a substantial portion of the total NOX formed in a pulverized coal flame.
The reactions involved in the formation of thermal NOX are generally regarded to be:
(1) O2=O+O PA1 (2) O+N2=NO+N PA1 (3) N+O2=NO+O PA1 (4) N+N=N2
Reaction 1 is an equilibrium reaction and the atomic oxygen formed in this reaction is in equilibrium with the molecular oxygen (O2). The relative equilibrium concentrations of Reaction 1 is very temperature dependent and the amount of atomic oxygen is very small below 2800 degrees Fahrenheit. Also, the total amount of atomic oxygen is dependent upon the concentration of molecular oxygen in the combustion zone.
Atomic oxygen formed in Reaction 1 can react with molecular nitrogen to form NO and N, as shown in Reaction 2. Atomic nitrogen, which is formed in Reaction 2, is converted at an efficiency of 5 to 50 percent to NO, as shown in Reaction 3, depending upon the availability of molecular oxygen in the combustion zone. If the concentration of molecular oxygen is low, then the dominate reaction for atomic nitrogen will be Reaction 4 that results in molecular nitrogen (hereinafter N2). N2 is the desired reaction product. These reactions have been studied, described, and quantified by Zeldovich. Zeldovich, Ya. B." Acta Physicohin, USSR, 21, 577. Therefore, to avoid thermal NOX formation, it is important to control the amount of coal that is burned in the combustion zone at temperatures above 2800 degrees Fahrenheit and to minimize the amount of excess oxygen in the combustion zone.
Fuel NOX is formed when fuel-bound nitrogen reacts with atmospheric oxygen. Fuel-bound nitrogen becomes atomic nitrogen (or part of a very reactive radical) when oxygen consumes the hydrocarbon molecule in which the fuel-bound nitrogen was originally located. Once atomic nitrogen become available in the combustion zone, it can react with molecular oxygen (Reaction 3) or it can react with another atomic nitrogen (Reaction 4). Reaction 3 is favored and NO is formed at efficiencies up to 50 percent, if there is excess air (which results in excess oxygen) present in the combustion zone. However, if there is little or no excess oxygen when the atomic nitrogen is liberated from the fuel, then Reaction 4 is favored and N2 is formed at efficiencies up to 90 percent.
Fuel-bound nitrogen contained in the volatile fraction of coal will be burned quickly because the volatile fraction of coal is evolved and burned within the first 200 milliseconds of combustion. This first 200 milliseconds represents the period in which atomic nitrogen from fuel-bound nitrogen in the volatile fraction is available for reaction. Therefore, to avoid fuel NOX formation, it is important to minimize or eliminate the amount of excess oxygen in the combustion zone where atomic nitrogen is formed.
NOX emissions from furnaces have been the subject of regulatory scrutiny for many years. Many successful devices and procedures have been used to reduce NOX emissions from furnaces. Fuels such as natural gas have no fuel-bound nitrogen and NOX emissions can be reduced by lowering flame temperatures. Reduced air preheat, flue gas recirculation and water injection have been used in various types of furnaces to reduce NOX emissions from natural gas combustion. However, these techniques are not effective in reducing the formation of fuel NOX. Oil fuel, which has some fuel-bound nitrogen, has sometimes been treated with the techniques used in natural gas combustion, but they are only partially effective.
The content of nitrogen by weight of coals typically burned by utilities can vary from 0.3% to over 2.0%. A coal having 1% nitrogen by weight and a heating value of 12,000 Btu per pound would emit the equivalent of 0.5 pounds of NOX per million Btu's, if only 20% of the fuel-bound nitrogen was converted to NOX. Any thermal NOX would add to this amount. Therefore, to meet expected emission limits and current limits for some furnaces (0.5 pounds of NOX per million Btu's of heat input) it is necessary that no more than 20 percent conversion of the fuel-bound nitrogen be converted into NOX. Numerous techniques have been tried to achieve these goals.
Slowly mixing or controlled mixing burners have been used on face fired and tangential fired furnaces to reduce NOX emissions. While some success has been achieved with these method, they are expensive and may result in increased carbon in the fly ash. Increased fly ash carbon can disrupt the functioning of the particulate removal devices and may cause destructive and dangerous fires in the back end of the combustion device. Controlled mixing burners have also been tried on roof-fired furnaces, but their application has been limited.
The roof-fired design which is of primary concern to the present invention uses multi-nozzle, inter-tube burners. The roof-fired design represents a relatively unique style of furnace that was designed and constructed in the late 1940's and early 1950's. The nitrogen oxide emissions from these units have not been extensively studied by applicants, but the emissions are believed to above levels allowed by current or imminent regulations. Existing NOX reduction technology can not be easily applied to these roof-fired units. A retrofit using existing NOX reduction technology is expensive, costing approximately six to seven times the cost of a conventional wall-fired furnace retrofit. Consequently, there is a need for a combustion apparatus and method which will both reduce nitrogen oxide emissions in flue gas and which can be readily used in existing roof-fired furnaces.
Many roof-fired furnaces have uniquely designed fuel delivery and burner systems. In these systems, coal is pulverized or milled so most of the coal will pass through a 70 mesh screen. The milled coal is then blown into the furnace by 10 to 25 percent of the combustion air. The coal and air from the pulverizer is divided into several pipes, each pipe supplying a burner which is typically 12 to 48 inches in diameter. This coal pulverization and delivery system is typical of many furnaces, but in some roof-fired furnaces the coal burner is further divided into 4 to 16 nozzles before the air and coal is discharged into the furnace. The burners are located in the roof of the furnace and the fuel is fired vertically downward. Different furnaces will have different numbers of pulverizers, burners, and nozzles per burner. These nozzles are only about 1 to 3 inches in diameter. The secondary air also is supplied through opening which usually are not more than 4 inches wide. Typically, there are multiple secondary air openings for each nozzle. The small size of these nozzle and secondary air openings allow the coal, primary air, and secondary air to be discharged into the furnace through spaces in between boiler tubes in the roof of the furnace. This type configuration is known as a multi-nozzle, inter-tube burner.
To retrofit roof-fired furnaces which currently employ the multi-nozzle, inter-tube burner with new lox NOX burners requires substantial modification to the furnace roof. The furnace top for roof-fired furnaces is usually defined by boiler tubes between which there are spaces. The nozzles and secondary air pass through these spaces. These tubes must be cut out and replaced with bent sections to allow new low NOX burners to be installed. This can be an expensive retrofit.
Another type of retrofit is the addition of NOX ports or overfire air ports. Typically, low NOX burners are installed in combination with overfire air ports. With overfire air ports, some combustion air is diverted from the burners and supplied to the overfire air ports. This results in the early stages of combustion (about 0.2 to 0.5 seconds) occurring in a fuel-rich environment. Because fuel-bound nitrogen contained within the volatile portion of coal is generally evolved during the first 200 milliseconds of combustion, the overfire air enters the combustion process after this fuel-bound nitrogen has been liberated. Because this fuel-bound nitrogen is liberated in a fuel-rich environment, it will preferentially react with atomic nitrogen to form N2 and will not react with molecular oxygen in significant amounts to form NOX. Further, because of the delayed addition of combustion air from the overfire air ports, the average combustion temperature has been reduced by heat transfer to the boiler tubes. This lowering of the combustion temperature will reduce thermal NOX formation.
However, the system just described has numerous drawbacks when applied to a roof-fired unit that uses nozzles to discharge coal into the furnace. Installation of the low NOX burners and overfire air ports requires modification and replacement of many boiler tubes in the furnace roof. The wind box must be converted to accommodate new and expensive low NOX burners. Duct work must be installed to bring overfire air from existing duct work or the windbox to the overfire air ports. Refractory throats must be constructed for both the burners and the overfire air ports. Dampers must be installed for the overfire air ports. Typically, when overfire air ports are installed, there is no easy method of adjusting the distribution of combustion air to assure substantially complete combustion while achieving the required level of NOX reduction.
As shown above, economical methods of retrofitting low NOX systems to roof-fired furnaces using multi-nozzle, inter-tube burner are not generally available. Such systems as are available have experienced only limited testing with natural gas, fuel oil, and pulverized coal.
Various back end or later furnace treatments to reduce NOX after it has been formed during combustion are available and are used in certain situations. One process is referred to as thermal deNOX, non-catalytic deNOX, or selective non-catalytic NOX reduction (hereinafter SNCR). Another process is referred to as selective catalytic NOX reduction (hereinafter SCR). Both of these require ammonia (hereinafter NH3), a toxic and difficult to handle gas or pressurized liquid. SNCR requires very careful injection of vaporized and diluted ammonia at a very narrow temperature window which may move in the furnace as load or other conditions change. SCR requires a very expensive catalyst. These systems are so expensive as to be practical only where the most stringent laws are in force and after the less expensive measures to reduce NOX formation during combustion have been taken. Further, these deNOX processes are usually applied to furnaces which only fire natural gas or oil.
Reburn, or in-furnace NOX reduction, is a technique where a fuel, usually natural gas or other high grade and expensive fuel which contains little or no fuel-bound nitrogen, is introduced in the furnace well downstream of the burners. The fuel is introduced in sufficient quantities to cause the gas stream to be fuel-rich. Temperatures of about 2000.degree. F. to 2400.degree. F. are desirable for this process but they are not always available before the gases flow through the convective passes of the furnace. The NO in the gas stream reacts with the fuel to form carbon dioxide, water vapor, molecular nitrogen, and fixed nitrogen compounds, such as, ammonia, hydrogen cyanide, and amines. Then enough additional air is provided to complete the combustion substantially and to make the gas fuel lean, preferably at the lower end of the temperature range. The fixed nitrogen compounds are oxidized to NO, and molecular nitrogen. Through this process the NOX is reduced by about 50%. The process is expensive to implement and reburn fuels are more expensive than coal. Additionally, many furnaces do not have sufficient volume to accommodate reburn.
Some efforts have been made to use remote or staged combustion to reduce NOX emissions. For example, Kochey, U.S. Pat. No. 4,316,420, discloses the introduction of a greater portion of the combustion air flow at location remote from where the fuel is initially burned.
Michelson, et al., U.S. Pat. No. 4,629,413, discloses blocking off secondary air ports near the fuel burner and reintroducing the secondary air at a remote location.
Hellewell, et al., U.S. Pat. No. 5,020,454, discloses the use of overfire air nozzles to inject overfire air at location remote from the coal burner.
And Yap, U.S. Pat. No. 5,229,929, discloses the use of secondary air nozzles to achieve staged combustion.
None of these prior art patents disclose an economic means of retrofitting roof-fired furnaces to reduce NOX emissions in the same manner as the present invention.